This invention relates to a method for forming a cold spot region on a discharge tube of a discharge lamp. The invention further relates to a discharge lamp with a cold spot region, where the cold spot region is constituted by a tubular extension located at an end of the discharge tube.
A wide variety of low pressure discharge lamps are known in the art. The majority of such lamps are so-called compact fluorescent lamps. These lamps comprise a discharge tube. The internal surface of the discharge tube is covered by a luminescent material, usually referred to as phosphor, also commonly termed as light powder. The phosphor emits a visible light when excited by UV radiation. The UV radiation is generated by the interaction of a mercury gas fill in the discharge tube, and the electric discharge between two electrodes. For this purpose, certain low pressure discharge lamps contain small doses of mercury. In order to achieve maximum light output, it is required that the mercury vapour is adjusted and stabilised at a well-defined partial pressure. This is possible by forming a so-called cold spot or cold chamber on the discharge tube, and by selecting an appropriate temperature in the cold spot, which is the coldest location of the gas discharge tube during operation of the lamp. Excess mercury condenses in the cold spot, automatically regulating the partial pressure of the mercury. In this manner, the temperature of the cold spot influences the partial pressure of the mercury in the discharge tube, which in turn directly affects the light output of the lamp.
Generally, compact fluorescent lamps having mercury-filled discharge tubes are tuned to provide maximum ligth output with a cold spot temperature of 40–45° C. The cold spot region of the lamp is normally designed to be on a part of the discharge tube which is relatively far from the driving electronics of the lamp, which latter tend to generate excess heat. For example, it is customary to form a cold spot region on the top of the discharge tube. However, this results in compact fluorescent lamps which may loose up to 20% of their light output in the base-down position (i.e. when the lamp base is below the discharge tube), as compared with the base-up position, because the ascending heat from the electronics and the discharge tube heats the cold spot region of the lamp, and the temperature of the cold spot increases to unacceptable levels.
U.S. Pat. No. 4,549,251 discloses a discharge lamp having a discharge tube bent to a special form. The discharge tube is provided with a long tubular extension at one of its ends. This extension serves as a cool region for the condensation of the mercury. The tubular extension is a remaining part of an exhaust tube, which latter is used to evacuate the discharge tube during manufacture. The exhaust tube is tipped off with a solid glass tip-off. It is explained in the U.S. Pat. No. 4,549,251 that the length of the exhaust tube is chosen to provides an optimum temperature of the cold spot.
U.S. Pat. No. 4,329,166 discloses an automatic tipping-off apparatus which is specially designed to perform the tipping-off of exhaust tubes of low pressure discharge lamps. Such an apparatus is capable of providing a hermetic sealing of the exhaust tube with an approximate wall thickness of 1 mm. This known apparatus is expressly designed with the aim of providing a uniform thickness of the tip portion of the exhaust tube. It is not taught or hinted that a non-uniform thickness of the exhaust tube could be advantageous.
It has been found that such known methods of providing a cold spot region are not satisfactory for compact fluorescent lamps which are designed to operate in a base-down position, and where the ends of the discharge tube are hidden within the lamp housing. Even with improved ventilation of the lamp housing, the wall thickness of the exhaust tube does not allow sufficient dissipation of the heat from the cold spot. Firstly, there are practical limits to the length of the exhaust tube, as a longer exhaust tube will tend to brake off during manufacture or other handling of the lamp. Secondly, even with a relatively long exhaust tube, the thermal load from the discharge volume is higher than the heat dissipation through the glass wall of the exhaust tube.
Therefore, there is a need for a discharge lamp with a more efficiently cooled cold spot region, which allows the operation of the lamp in a substantially arbitrary position, without any significant loss of the light output. There is also need for a method for manufacturing such a discharge lamp. It is sought to provide a method, which, beside providing the required efficiently cooled cold spot region, is relatively simple and which does not require expensive components and complicated manufacturing facilities, and which may be integrated into various types of existing production lines in a straightforward manner.